In the navigation of an aircraft or other vehicle over a planned route, contour maps are typically relied upon to indicate the configuration of the terrain over which the aircraft passes, and the pilot of the aircraft utilizes the data provided by such contour maps in conjunction with instrument readings and visual observation of the terrain in determining the altitude and course of the aircraft as it passes from point to point along the route. However, for low altitude flying, such as might be encountered in a helicopter or other low flying aircraft, for example, an instantaneous indication of the details of the terrain over which the aircraft passes is essential to the quick reaction required in the guiding of the aircraft over a terrain which may provide rapidly changing contours and other obstacles to the flight of the aircraft.
Weather conditions which result in poor visibility for an aircraft or over-land vehicle also hinder the use of simple contour maps for purposes of navigation Thus, the problems which have been experienced to date in the navigation of helicopters at so-called nap-of-the-earth (NOE) altitudes have been formidable especially under conditions of limited visibility. Even where visibility is not a factor, the navigation problems with NOE flight do not have simple solutions. Thus, if a map were available which could readily identify the instantaneous position of the aircraft or vehicle and display the terrain in the immediate vicinity of the aircraft, and if the map would always be oriented in the direction of the aircraft heading such that upcoming terrain could be easily identified, and if any additional information the pilot required was accurately identified thereon, then the NOE effectiveness of the pilot would be greatly enhanced.
Accordingly, various systems have been proposed heretofore, including radar scanning systems and systems using preprocessed films of terrain over which an aircraft is to pass, for providing to the pilot a display which simulates that which he would visualize if he were able to actually view the terrain over which the aircraft is passing. Unfortunately, such systems have not been entirely satisfactory in that they are often quite complex and are not capable of providing the detail insofar as elevation and cultural data is concerned which is required by the pilot of the aircraft for proper guidance. Further, in systems in which preprocessed films are used to provide the navigational display data, only a flight over a specific path correlated to the information provided on the film is possible.
To overcome these problems, it has been suggested that a computer-controlled system including a large memory capacity be provided for the storage of terrain data in digital form However, such a solution requires an extremely-large memory capacity to store all of the data necessary for the display of both elevation and cultural features for a sufficiently-large area to accommodate the required maneuverability of the aircraft. In addition, such systems typically require a scene or display memory for storage of the data to be displayed, and the update or refresh of this memory to accommodate changes in display data in response to movement of the aircraft and changes in heading thereof have resulted in poor picture quality due to the slow speed of these update or refresh operations.